Two bacterial selenium-containing proteins that participate in oxidation-reduction reactions are under investigation in the laboratory. One of these, the small selenoprotein of the glycine reductase system, is particularly useful as biological material for investigation of the chemical and biological properties of this novel type of catalyst. The chemical stability of the protein renders it suitable for studies on amino acid composition, end group analysis, characterization of the active site and identity of the selenium containing moiety that are currently in progress. Antisera prepared to the pure protein also are to be used to characterize native and derivatized forms of the selenoprotein and selenium-containing peptides derived from it. The selenium-containing formate dehydrogenase of a methane- producing organism, Methanococcus vannielii, and of Clostridium sticklandii is also under study as regards nature of its selenium- containing moiety, biological role of selenium in the enzyme and presence of other trace elements.